1. Field of the Invention
The present invention relates to data transmission apparatus adaptive to data quality on radio-transmission and a method of data transmission for the same, and more particularly, for example, to such apparatus operative on the Bluetooth (trademark) function and such a method of selecting types of packets on the basis of CQDDR (Channel Quality-Driven Data Rate) function on Bluetooth radio-transmission to transfer packets of data.
2. Description of the Background Art
Conventionally, an example of radio-transmission system functioning on the basis of Bluetooth includes one master device and at most seven slave devices communicable with the master device over a link of piconet. On that type of radio-transmission, data are transmitted with the throughput optimized. The optimization of data transmission is presently regulated by the Bluetooth Specification, Ver. 1.1, Part C, pp. 223–224, May 8, 2001. The Specification also regulates the function of CQDDR (Channel Quality-Driven Data Rate). In the CQDDR function, each of the devices receives and measures data packets transmitted on a radio-transmission pass, referred to as an RF (Radio Frequency) channel, to evaluate a transmission quality, from which on the basis of the result the receiver devices request the device which will transmit data packets to selectively control the types of data packets to be transmitted.
On data transmission, an asynchronous connectionless link (ACL) is employed. Generally, two types of packet are selectively transmitted on an ACL link independence upon whether or not to function with ⅔ FEC (Forward Error Correction). The CQDDR function controls a transmitter device to select either of the two types of packet in dependence upon the transmission quality measured to adaptively switch the data rate on the piconet.
Such adaptive switching of a data rate is adopted by a communication device disclosed, for example, by Japanese Patent Laid-Open Publication No. 2002-290415. The communication device disclosed by the Japanese publication works on the Bluetooth so that audio signals may be dealt with in the form of SCO (Synchronous Connection Oriented) packets to establish communications. For that application, types of packets available include HV1, HV2 and HV3 packets.
Those packets regulated will briefly described. Basically, SCO packets are not re-transmitted. An HV1 packet includes only a payload body, in which ten bytes of user data are included. The 10-byte data do not include an error correction code under the cyclic redundancy check (CRC) system. Instead, data are error-correction-encoded at ⅓ rate, i.e. encoded on Rate ⅓ FEC (Forward Error Correction), to ultimately assemble the packets having the payload length of 30 bytes, i.e. 240 bits. An HV2 packet includes 20 bits of user data. Data are error-correction-encoded at ⅔ rate, i.e. encoded on Rate ⅔ FEC. An HV3 packet includes 30 bits of user data, and not an error correction code.
Under the Bluetooth Specification, various types of transmission packets are regulated. It is however not regulated which types of packets are selected under which conditions. The telecommunications apparatus exemplified above are desirous to establish, with the Bluetooth Specification satisfied, transmission most appropriate for a telecommunications environment as well as minimum consumption of transmission power. The exemplified telecommunications apparatus are adapted to set up an HV3 packet when commencing transmission, to detect the frequency of transmission errors encountered in a unit period of time during communication, and to select an HV2 or an HV3 packet adaptively to the detected frequency so as to satisfy the requirements regulated to continue transmission.
The Bluetooth Specification also regulates packets to be transmitted on the ACL link. Nothing is however regulated on how to measure, in a piconet established, the quality of an RF channel at slave devices which are going to receive packets, nor a quality standard for use in controlling, in accordance with a result from the measurement, a master device, i.e. a remote device, to select an appropriate type of packets. This in practice renders designers' discretion specific for Bluetooth devices designing the detailed specifications of the CQDDR function stated earlier.
Devices receiving data have in fact a CQDDR function for general purpose measuring errors in received data on the basis of the Bluetooth Specification to compare a measurement with a predetermined threshold to determine how the quality of data transmission is. When a device receiving data often detects an error frequency falling in the vicinity of the threshold, it issues a message requesting a remote device for switching the type of packets to another. This results in increasing a data transmission rate on the piconet so as to enhance the data throughput on that remote device.